Studying downward TGFs with the largest ground array of gamma-ray detectors

<p>The Pierre Auger Observatory, the largest cosmic-ray detector in the world, has been<br />observing peculiar events which are very likely downward TGFs. Their experimental<br />signature and their time evolution are very different from those of a shower produced<br />by an ultra high energy cosmic ray. The TGF-like events happen in coincidence with<br />lightning and low clouds and their deposited energy at the ground is compatible with<br />that of a standard downward TGF with the source at few kilometers above the<br />ground. The surface detector (SD) of the Auger Observatory consists of 1660 water-<br />Cherenkov detectors (WCDs) spread over 3000 km² in the Argentinian pampa. The<br />WCD height of 1.2 m makes them highly sensitive to gamma rays and the large area<br />covered with SD allows us to sample the TGF beam from different points. The<br />timing shape of WCD signals can be very important to constrain different TGF source<br />models. Cold runaway from the high fields near the leader tips or relativistic<br />feedback produce the same energy spectrum but predict a different rise and fall of the<br />counts versus time, and they could produce a different angular distribution.<br />Comparisons between simulations and data will be shown.<br />Moreover, first results from a preliminary analysis of the available meteorological<br />data at the time of Auger TGF-like events will be presented. Little is known about the<br />TGF-producing storms. The characteristics of these thunderstorms are being<br />investigated by studying meteorological data in coincidence with upward TGFs. A<br />similar analysis is important to better understand downward TGF production<br />mechanisms and investigate if are the same as those producing upward TGFs.</p>